1. Field of the Invention
This invention includes two parts: (1) a method for producing amorphous Co-Tb perpendicular magnetic recording thin films, and (2) a method for producing amorphous Co-Tb longitudinal magnetic recording thin films.
2. Description of the Prior Art
Recently, the amount of information storage has increased rapidly due to the rapid development of the computer industry. In order to get high recording density, the magnetic recording media have been improved from the conventional y-Fe.sub.2 O.sub.3 particle system to the thin film media. A high recording density thin film medium needs high coercivity (Hc) and optimum saturation magnetization (Ms) for MR and GMR magnetic heads.
The most important problem in recording medium is how to increase recording density. At present, the CoCrM (M=Ni, Ta, Pt) crystalline thin films and columnar grains CoCr films are the most widely used longitudinal and perpendicular magnetic recording materials respectively, due to their high coercivity (Hc=1500.about.2400 Oe). For these crystalline films, the most significant problem is the noise that results from magnetic exchange coupling between grains located at domain transition regions (Jian-Gang Zhu, "Transition Noise Properties in Longitudinal Thin Film Media", IEEE Trans. Magn. Vol.29, no.1, pp.195-200, 1993). Otherwise, intergranular voids, stacking faults, crystallographic orientation, etc., all will decrease magnetic performance (Shaun E. Mckinlay, Nina Fussing, and Robert Sinclair, "Microstructure/Magnetic Property Relationships in CoCrPt Magnetic Thin Films", IEEE Trans. Magn. Vol.32, no.5, pp.3587-3589, 1996). Practically, if we want to increase the areal recording density of the crystal film, the grain size of the film must be reduced (D. N. Lambeth, E. M. T. Velu, G. H. Bellesis, L. L. Lee, and D. E. Laughlin, "Media for 10 Gb/in.sup.2 Hard Disk Storage: Issues and Status", J. Appl. Phys., 78(8), pp.4496-4501, 1996). However, when the grain size of the film is smaller than single-domain size, grains will become superparamagnetic particles and coercivity of the film decreases rapidly due to thermal fluctuation. The recording bit size is limited by the single-domain size.
Although the fabrication of a thin film with single-domain nanocrystal particles could be achieved by various methods (J. Nakai, M. Kuwabara, A. Kikuchi, T. Sakurai, T. Shimatsu and M. Takahashi, "Effect of Microstructure on Media noise of CoCrTa Thin Film Media Fabricated Under Ultra Clean Sputtering Process", IEEE Trans. Magn. Vol. 31, no.6, pp.2833-2835, 1995 G. Choe, "Effect of Film Morphology on Grain Boundary Segregation Induced Magnetic Properties in Heat Treated CoCrPt/Cr Films", IEEE Trans. Magn. Vol. 31, no.6, pp.2809-2811, 1995 I. Kaitsu, A. Inomata, I. Okamoto, and M. Shinohara, "Magnetic Properties and Structure of (Co-alloy)-SiO.sub.2 Granular films" IEEE Trans. Magn., Vol.32, no.5, pp.3813-3815, 1996 G. Zangari and D. N. Lambeth, "Porous Aluminum Oxide Templates for Nanometer-sized Magnetic Arrays", lntermag"97, Paper FC-01, 1997), Gaussian distribution of particle size often occurs during the fabrication of thin films. It is difficult to obtain uniform single-domain particles. Some of the particles will be multi-domain particles and some of the others will be superparamagnetic particles. Moreover, the distance between the particles is uncontrollable. Each bit of data is stored over at least several particles and the recording density is difficult to increase.
For overcoming the disadvantages described above, we invented those using high coercivity amorphous Co-Tb thin films as longitudinal and perpendicular magnetic recording media. If the read-write ability of a magnetic head is strong enough, the recording bits of this amorphous film can be reduced to uniformly isolated single-domain size with the desired shape and smallest distance between neighboring recording bits without grain boundary or crystallographic orientation problems. Thus, the recording density could be increased enormously. The high coercivity amorphous Co-Tb thin films produced by this invention may be one of the most promising candidates for future high-density magnetic recording either in longitudinal recording or perpendicular recording. For the as-deposited amorphous Co-Tb thin film, its perpendicular coercivity (Hc.sub..perp.) is higher than 6000 Oe and its saturation magnetization (Ms) is higher than 100 emu/cm.sup.3. This film can be used as a perpendicular magnetic recording medium for ultra-high density magnetic recording. For the annealed amorphous Co-Tb thin film, it has nearly magnetic isotropy properties. Its inplane coercivity (Hc.sub..parallel.) is equal to 2080 Oe and its saturation magnetization (Ms) is about 100 emu/cm.sup.3. This film can be used as longitudinal magnetic recording medium.
Amorphous Co-Tb films have been studied by many investigators. They were all applied in magneto-optical recording. Until now, the amorphous TbCo thin films have never been used in magnetic recording. The magnetic properties demanded for magneto-optical recording media are different from that of magnetic recording media. According to the literature (T. Niihara, S. Takayama, K. Kaneko, and Y. Sugita, "Perpendicular Anisotropy of Tb-Co Amorphous Films Sputtered in H.sub.2 -added Ar gas", Appl. Phys. Lett. 45(8), pp. 872-874, 1984; T. Niihara, S. Takayama and Y. Sugita, "Perpendicular Anisotropy in Tb-Fe and Tb-Co Amorphous Films Sputtered in H.sub.2 -added Ar gas", IEEE Trans. Magn. Vol.21, no.5, pp.1638-1640, 1985), they added H.sub.2 in Ar sputtering gas for improving perpendicular anisotropy of amorphous TbFe/TbCo films, their Tb contents range from 20 at. % to 30 at. %. They investigate the correlation between H.sub.2 partial pressure, perpendicular anisotropy constant (Ku), saturation magnetization (Ms), and internal stress of the film. The coercivities of their films are not reported, and their sputtering gas is a mixture of H.sub.2 and Ar gases that is different from this invention (in this invention, the sputtering gas is pure Ar gas). Their films are used in magneto-optical recording and our films are used in magnetic recording, the applications are different. The other literature (Seiji Yoshino, Hiroshi Takagi, and Shigeru Tsunashima, Mroio Masuda and Susumu Uchiyama, "Perpendicular Magnetic Anisotropy of TbCo Films", Japan. J. Apply. Phys., 23, pp.188-191, 1984) studied the perpendicular anisotropy constant Ku of the TbCo films prepared at substrate bias voltage of -100 volts. They found that Ku decreased considerably after annealing. Tb contents of their films are lower than 25 at. %, the Ms values and the coercivities of these films are not reported. In this invention, the substrate is not biased and Tb content of the Co-Tb film must be higher than 25 at. % in order to get high coercivity. Another literature (M. Ohkoshi, M. Harada, T. Tokunaga, S. Honda and T. Kusuda, "Effects of Ar Pressure and Substrate Bias on Magnetic Properties and Microstructure in Amorphous TbCo Sputtered Films", IEEE Trans. Magn. Vol.21, no.5, pp.1635-1637, 1985), amorphous TbxCo.sub.100-x (x=19,21,24), describes that films were prepared by r.f. diode sputtering with various Ar pressures and the substrate is biased with a negative voltage between 100 and 150 volts. They investigate the correlation between perpendicular anisotropy constant (Ku), saturation magnetization (Ms), sputtering Ar pressure, and biased voltage. These films all exhibited perpendicular anisotropy and were applied in magneto-optical recording. The coercivities of these films are not reported and Tb contents of the films are lower than 25 at. %. In this invention, if we want to get high coercivity, Tb content of the Co-Tb film must be higher than 25 at. % and the substrate is not biased. Our amorphous Co-Tb films are applied in magnetic recording. Further investigations (S. Honda, M. Ohkoshi and T. Kusuda, "Change of Magnetic Properties in Compositionally Modulated TbCo Sputtered Films", IEEE Trans. Magn. Vol.22, no.5, pp.1221-1223, 1986) studied the compositionally and morphologically modulated TbCo films that were annealed at 300 for 6 hours in a vacuum. Their substrate is biased with an alternately negative voltage between 0 and 100 volts. The films have 12-36 layers (layer thickness is 80-250 .ANG.). They found a maximum coercivity of about 7 kOe at Tb=20.3%, its Ms value is 30 emu/cm.sup.3. However, its microstructure has been changed due to oxidation during long period and high temperature annealing. Since their TbCo film is annealed at 300 for 6 hours, it may be crystallized. Because the crystallization temperature of amorphous TbCo film is about 270 (P. Hansen, in "Handbook of Magnetic Materials", ed. by K. H. J. Buschow, (Elsevier Sci. Publ. B. V., 1991), Vol.6, Chap. 4, p.310). Their TbCo films are nanocrystalline films and not pure amorphous films. In this invention, the substrate is not biased and the annealing temperature is 250.degree. C. which is lower than the crystallization temperature of amorphous Co-Tb film. So, the structure of annealed Co-Tb film is still an amorphous structure. And the Ms value of the amorphous Co-Tb film is equal to 100 emu/Cm.sup.3.